Description of the Prior Art
The present invention is directed to a position measuring device and more particularly to a position measuring device using an integrated optical sensor arrangement.
Position measuring arrangements are used with machine tools, for example, to measure the relative position of a tool with respect to a workpiece being processed by the tool. U.S. Pat. No. 4,938,595 based upon German Letters Patent DE-PS 36 25 327 discloses a photoelectric position measuring arrangement for measuring the relative position of two objects. A scale connected with one object and equipped with a diffraction grid is scanned by a scanning unit having a semiconductor laser. An integrated optical device located on a substrate is connected with the other object. The integrated optical device includes two coupling elements in the form of two horn-shaped diffraction grids located at two input waveguides of a waveguide coupler. Also included are three detectors located at three output waveguides of the waveguide coupler. The light beam bundle emitted from the semiconductor laser is diffracted by the diffraction grid of the scale into two diffraction beam bundles of the same order but with opposite signs. The diffraction beam bundles enter the two input waveguides by means of the two coupling elements. The diffracted beam bundles interfere with each other in the waveguide coupler and subsequently impinge the three detectors located at the three output waveguides. Periodic scanning signals are generated from the detectors, the signals being phase displaced with respect to one another. The measured values for the relative position of the two objects are obtained from these signals in a manner known to those skilled in the art.
Accurate operation of an integrated optical device of this type requires the emission wavelength of the laser, the grid parameters of the coupling elements, the diffraction indices of the waveguides and the ambient media to be adjusted with respect to each other. To assure accurate operation, it is necessary to set an optimal emission wavelength in the laser and to maintain this optimal emission within a tolerance of 10.sup.-3 .lambda.. This optimal setting is disturbed, however, if there is a change in the ambient conditions particularly temperature changes. Disturbance of the optimal emission wavelength are undesirable since it results in measurement errors.
A device for controlling or regulating the emission wavelength and the emitted optical output of a semiconductor laser is known from U.S. Pat. No. 4,815,081 based upon EP-A1-0 284 908. The control or regulation of the emission wavelength and the emitted optical output is performed simultaneously. The optical output emitted by the laser is partially fed to a first detector arrangement and subsequently, via at least one Bragg grid or a directional coupler acting as filter element, to a second detector arrangement. The two detector arrangements simultaneously measure the optical output and the emission wavelength of the laser and simultaneously generate therefrom two control signals for the injection current of the laser.